Aluminium smelters, comprising many aluminium reduction cells (colloquially called “pots”), produce large amounts of hot flue gas containing hydrogen fluoride, other gases, particulates, and sublimates. For example, an aluminium smelter producing 300,000 tonnes/year of aluminium metal will comprise about 400 reduction cells, arranged in four rows. Such a potline will produce about 3,600,000 Normal cubic meters per hour (Nm3/h) of flue gas. The raw flue gas must be cleaned, and a well-known type of gas cleaning process used in this connection is the so-called “dry scrubbing” process. Gas cleaning plant using this process is available, for example, from ALSTOM Power Environment Control Systems at Drammensveien 165, 0277 Oslo, Norway (Tel. +47 22 12 70 00).
The trend of technical development in the aluminium production industry is toward the hooded pre-bake type of reduction cells. These are increasing in size and energy input, resulting in increasing flue gas temperatures. From a previous level of 70-90° C., the flue gas temperature from the most modern potlines is now in the range 120-180° C., or even more. Unfortunately, such flue gas temperatures exceed acceptable temperature levels for gas cleaning plant using the dry scrubbing process, with regard to both the process and the equipment. Consequently, the dust- and impurity-containing raw flue gas has to be cooled before it enters the cleaning plant.
It is known to cool hot raw flue gas produced by aluminium reduction cells by mixing cool ambient air into the flue gas ducts upstream of a gas cleaning plant. Gas/air mixing is easy and cheap, but when gas temperatures are up to or exceeding 150° C., the volume of ambient air required to give adequate cooling becomes substantial, and so does the increase in cooled gas volume. Hence, there is a corresponding increase in the size of the gas cleaning plant, the downstream fans that pull the gas through the cleaning plant, and the plant energy consumption. This adversely affects the plant economics, both during its construction and during its operation.
It is also known to cool the flue gas from aluminium reduction cells by evaporation using direct injection of atomised water. Although this system reduces the overall flue gas volume, the volume of steam thereby produced must also be taken into account. Direct cost for this cooling system is moderate, but it requires large quantities of compressed air for atomisation of the injected water, so energy consumption for air compression is high. Furthermore, the system requires quite large quantities of fresh, purified cooling water, which is an economic and environmental disadvantage in areas where fresh clean water is a valuable asset. Additionally, assuming filter bags are used in the subsequent gas cleaning plant, high humidity in the cooled flue gas may hydrolyse the standard polyester type of filter bag, necessitating use of a more chemically inert and very considerably more expensive material for the filter bags.
In some existing aluminium smelting plants, it is known to recover heat from the flue gas after it has been cleaned by passing it through heat exchangers to produce warm water for heating purposes. Hitherto, such heat recovery has only been possible after the gas has been cleaned, because dust, sublimates and other impurities in raw flue gases would otherwise tend to deposit as hard scale onto heat exchanger surfaces, resulting in clogging and reduced heat transmission in the heat exchanger. Moreover, at present the building of new or larger aluminium smelting capacity is mainly limited to tropical or subtropical countries, in which the energy requirement for heating purposes is limited or non-existent, the greater need usually being energy for cooling purposes.
It is known to use the gas tube type of gas cooler to cool hot flue gases from steel, ferro-silicon and silicon metal furnaces. In such coolers, the flue gas—which comes from the furnace to the cooler via flue ducting—flows through a bundle of parallel tubes, with the coolant flowing over the outside of the tubes. The fumes and particulates in these gases tend to form insulating layers of dust on the cooler tube walls that reduce the heat transmission in the coolers. This type of dust layer can be blown down to an acceptable thickness and can be nearly completely removed by maintaining a high gas velocity along the tubes, thus maintaining an acceptable heat transmission in the coolers.
The fumes and particulates in the flue gas from aluminium reduction bells have, in contrast to the fumes and particulates in the above-mentioned furnace gases, a strong tendency to form hard-as-stone scales when impacting surfaces in turbulent gas flow zones, and surfaces that lie across the gas flow. These scales are too hard and compact to be blown away and removed by any practical gas velocity in tubes or other parts of a gas transportation system. Besides having an insulating effect in a gas cooler, such scaling tends to continue to grow on impact surfaces and in zones of turbulence until the cooler is completely clogged.
It will therefore be understood that the above-mentioned gas tube type of gas cooler, in the form used to cool hot flue gases from metal furnaces, is unsuitable for use in cleaning flue gas from aluminium reduction cells.